High power relativistic magnetrons are devices that emit microwave radiation in short pulses. These devices are plagued by several problems. Because these devices operate in a pulse mode and the pulse lasts only a few hundred nanoseconds, any reduction in the amount of time it takes to achieve mode lock significantly increases efficiency. It is not uncommon for current devices to take 150 nanoseconds or more to lock into a mode, and even then it may not be the desired energy-efficient pi mode. Another problem is mode competition, particularly for higher values of axial magnetic field. The excitation of modes other than the pi mode results in excessive noise and a reduction in output power and efficiency. These problems have precluded operation of relativistic magnetrons at high magnetic fields, which are theoretically more energy efficient and capable of producing sustained power output in excess of a gigawatt.
Several methods have been used to prime magnetrons by external means to preferentially excite the device in the desired pi operating mode. This can result in faster oscillation startup, elimination of mode competition, and frequency locking. Radiation priming involves injecting a low level external signal at the same frequency as the desired operating mode. A simpler and less expensive technique is cathode priming. In this technique, the cathode is fabricated by ablating azimuthally periodic emitting regions on the cathode by a KrF laser. (M. C. Jones, V. B. Neculaes, Y. Y. Lau, R. M. Gilgenbach and W. M. White, “Cathode priming of a relativistic magnetron,” Appl. Phys. Lett. 85, pp. 6332-6334, December 2004.) Another technique is magnetic priming, which uses an azimuthally-periodic axial magnetic field of N/2 periods to rapidly pre-bunch the electrons into the desired N/2 extraction cavities for pure pi-mode operation in an N-cavity magnetron. (M. C. Jones, V. B. Neculaes, W. M. White, Y. Y. Lau, R. M. Gilgenbach, J. W. Luginsland, P. Pengvanich, N. M. Jordan, Y. Hidaka, and H. L. Bosman, “Simulations of magnetic priming in a relativistic magnetron,” IEEE Trans. on Elec. Devices, 52, pp. 858-863, May 2005.)